Tell me if this is good or dangerous...

[Antero]

Hello!

I have a 12v 7Ah small battery. I will go camping and I bought a solar panel to charge it or at least to keep it charged.

The solar panel is: Pm:5W Vmp:18V Imp: 0.278A Voc: 21.90 Isc: 0.30A.

Can I plug it directly to that battery?
Can I plug it directly to a car battery? (it has a lighter plug for that)

Or this is dangerous and I have to buy a charge controller?

or this is not useful because this solar panel would take ages to charge that battery?

Thank you for your help and patience.

Kind regards from Azores.

Antero.

[Cariboocoot]

Welcome to the forum.

It should not be a problem as it will not be connected permanently and it won't put out a lot of current.
You are correct that it won't do much for actually charging the battery; it would only be able to keep ahead of self-discharge.
If you had two such panels you'd probably do all right, depending on how much you discharge the battery.

[niel]

you could have a problem if you put the wires backwards. put a fuse in line just in case. use 1/2a.

[Cariboocoot]

Good point, Niel. I got to thinking it was a polarized connection due to the mention of the lighter socket hook-up but he's going to use it on a "free standing" battery so that's an issue.

Now I wonder if it has a blocking diode built in because that would be another issue: leave it connected all night and the battery can discharge through the panel.

I've got one of these little panels to keep the van battery up while not in use and it works fine: plugs into the lighter and produces enough to stay ahead of self-discharge without over charging or needing a controller. Mine has a blocking diode because it is designed just for this use. It's an older version of this: http://www.canadiantire.ca/AST/brows....jsp?locale=en

I see they have a 1.8 Watt "battery maintainer". Can't imagine that will do much for a full size car battery (about 0.15 Amps).

[Antero]

Hi!

Thank you for prompt answers.

It apeares to have no diod.

This is the one I bought: http://www.ebay.es/itm/221053511520?...#ht_4930wt_805

It says it can be plugged to a battery like mine. But one never knows...

Niel, what do you mean with putting the wires backwards? Invert polaritys?

Thank you!

[Cariboocoot]

Okay, so remember to disconnect it when not in use.

Most of the connectors for that are polarized. The only trouble comes when you connect to your free standing battery: get the (+) and (-) mixed and you have a problem (the panel will create a direct path for the battery to discharge through and it will do so quickly at full power, hence Niel's suggestion of a fuse to prevent the current from igniting the panel or wires).

Much of the success of this project hinges on how you connect the panel to your battery.

[BB.]

yep, connect a solar panel backwards to a battery, and instead of charging the battery, you will have a short circuit through the panel and destroy it.

So,.any plug connection should be polarized so that you connect plus to plus every time. All it takes is one oops.

-Bill

[Antero]

I see...

The only mistake I can make is with the clamps to the battery. All other plugs are polarized.

Where do i put the fuse? Positive? I Think I can't do it alone.

[Cariboocoot]

I see...

The only mistake I can make is with the clamps to the battery. All other plugs are polarized.

Where do i put the fuse? Positive? I Think I can't do it alone.

Yes; it's standard practice to fuse positive conductors on DC circuits. The fuse should be small, like 1 Amp; enough to handle the panel's output but will blow quickly if things go wrong.

If you get a polarized connector hardwired to your battery first it should greatly reduce the potential for any such problem. Once you have that wired right, it's plug-and-play.

Although we all know how well that works with Windows OS on computers, eh?

[BB.]

For such a system, it really does not matter where you put a fuse--Positive is the default location.

If you are clamping to the battery--Mark the positive post/area bright red so that it is obvious. Solar panels are actually a giant set of diodes, and if you connect them "backwards" (positive clip to negative battery terminal, etc.), they will conduct the full current of the battery. A fuse may not even help (although, a 1/2 amp fuse would not hurt).

Many small batteries have a nut and bolt. You could take any small polarized connector/plug from an electronics parts shop or possibly auto parts store (rated for 1/2 amp or more) and hardwire that to the battery and solar panel. That will reduce the chance of miss-connection.

-Bill